LibGUI: Fix ColorPicker custom color offset

Previously the ColorPicker would get the custom color directly from the
window, this was changed in d7d57884694ad5fda53e865f1b92ef48fddc9ca8 to
get the color from the underlying bitmap instead - without taking the
bitmap's scaling into effect though, so resulting colors were off by
quite a bit.

Fixes #3113.

Libraries/LibGUI/ColorPicker.cpp

@@ -418,7 +418,10 @@ void CustomColorWidget::pick_color_at_position(GUI::MouseEvent& event)
     if (!frame_inner_rect().contains(position))
         return;
 
-    auto color = m_custom_colors->get_pixel(position);
+    // Map actual event position onto scaled bitmap to get the right pixel
+    auto pixel_x = min(position.x() * m_custom_colors->width() / frame_inner_rect().width(), m_custom_colors->width() - 1);
+    auto pixel_y = min(position.y() * m_custom_colors->height() / frame_inner_rect().height(), m_custom_colors->height() - 1);
+    auto color = m_custom_colors->get_pixel({ pixel_x, pixel_y });
     m_last_position = position;
 
     if (on_pick)